Here's some possibilities.
1. One has another language ROM.
2. One has FCC testing, the other has CE or another RF emissions for the country's standards that matter. sometimes they need to tweak one over another. The effect may be invisible to the end user and will annoy them since you get model numbers and no apparent difference.
3. Warranty. You will not get service for an out of US product in the US. That's how it works.
Bob
What's the difference in the USA version and the international version of the same pda? I don't want to buy the international version and then find out something doesn't work right.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic